1.Field of the Invention
The present invention relates to a polarizing optical element, and particularly to an optical pickup system or lens system used in an optical data reading device.
2. Description of the Related Art
There is a demand for a bifocal lens having two different focal lengths usable for an optical pickup system or lens system in an optical data reading device such as a compatible video optical disc player and the like which can read data from a so-called CD (compact disc), SD (super density disc) and the like. It is convenient to play such optical discs of CD, SD and the like which are formed on the basis of different specifications such as various different optical disc thickness by only a single player device. In case of reproduction of data from the optical disc with a multi-layered structure, the thicknesses of layers to be focused are different in such an optical disc. Therefore layers of the multi-layered disc cause spherical aberrations to the objective lens for reading data. Further, the most suitable numerical aperture NA of a certain objective lens for reading data varies for the different optical discs. The bifocal lens is helpful for solving such problems.
A bifocal hologram lens is known. The hologram lens comprises a circular diffraction grating including concentric circular lands and grooves are having a refractive index distribution allowing for a lens action. The hologram lens can separate an incident light beam into some light beams because of its diffraction.
It is difficult to manufacture the bifocal hologram lens having a high diffraction efficiency. The bifocal hologram lens tends to reduce the light beam intensity practically used for the pickup device. In addition, since the performance of the bifocal hologram lens is dependent on the another incident light beam, the bifocal hologram lens is weak against deviations of another of the incident light beam source. These are drawbacks of the bifocal hologram lens.